The pleural, peritoneal and pericardial cavities contain organs that must be able to change size and shape and to slide within their serous cavities to function normally. Sliding is facilitated by extremely slippery and well-lubricated mesothelial surfaces which line the cavities and cover the organs. The mesothelial cells are delicate and easily destroyed by ordinary handling. It is not known how these surfaces are lubricated or how mesothelial cells are protected from frictional damage. We propose to explore hydrodynamic mechanisms that lubricate mesothelial cells, protecting them from high shear stresses associated with sliding movements, and that help maintain an adequate layer of lubricating liquid between the sliding serosal surfaces by regulating total liquid volume. Specific Aims are to 1) measure and characterize friction between sliding mesothelial membranes; 2) observe and characterize shear-induced smoothing of serosal surfaces; 3) establish the levels of shear stress that are damaging to mesothelial surfaces; 4) measure the topography of serosal surfaces to gauge surface roughness and to define the topographical location of stomata leading to lymphatic channels; and 5) to use computational fluid dynamics and finite element analysis to evaluate hypotheses related to our findings. These studies may reveal the extent to which mesothelial surfaces of lung and chest wall come in contact, and thus may support or disprove one of two contradictory views of pleural space geometry and mechanics.